Ultrasonic waves have become of great importance in recent years. Their unique properties have been applied to industry, signaling, medicine and many other fields.
The use of ultrasonic waves to treat and inspect molten aluminum is known, though not yet widely practiced commercially.
It is known, for example, that molten aluminum can be treated with ultrasonic waves of relatively low frequencies (15-20 KHz) and high power (0.1-several hundred watts) to achieve improvements in metal quality. The effects reported include degassing of the molten aluminum to decrease its hydrogen content, facilitated dispersion of alloying elements in the molten aluminum, and in respect to the solidified metal, grain refinement and increased workability and mechanical properties.
For the inspection of molten aluminum, relatively high frequencies (1-10 MHz) and low power (0.004-0.04 watts) are used. The most practical means of inspection is the pulse-echo method wherein an ultrasonic wave pulse is transmitted into the molten aluminum and the pulse reflections or echoes are detected and measured. Melt quality can be characterized in terms of the number and amplitude of the echoes reflected from discontinuties such as insoluble melt constituents, attenuations in pulse amplitude, pulse velocity through the melt, and shifts in the ultrasonic wave frequency.
Other applications of ultrasonic waves to the treatment or inspection of molten aluminum are of course possible.
For details concerning the implementation of ultrasonic wave technology generally, see B. Carlin, Ultrasonics, McGraw-Hill Book Company, Inc., New York-Toronto-London (1960), the disclosure of which is hereby incorporated herein by reference.
To transmit or receive ultrasonic waves to or from an aluminum melt, it is common to use an electromechanical transducer device for converting electrical energy to mechanical energy and vice versa. The most popular electromechanical conversion systems rely either on magnetostriction or the piezoelectric effect to operate. However, magnetostrictive transducers are not generally used for inspecting molten aluminum because of their characteristic low operating frequency (e.g. 60 KHz or less).
Piezoelectric transducers typically have the capability to both transmit and receive ultrasonic waves. Thus a single piezoelectric transducer may be used to perform both functions, or separate transducers may be used for transmitting and receiving. Piezoelectric transducers can readily be made to handle high frequencies and low power levels, and are accordingly well suited for molten aluminum inspection methods.
A transducer can conveniently be coupled to the melt using a probe, sometimes called a "delay line" or a "mechanical standoff". See, for example, U.S. Pat. No. 3,444,726 to R. S. Young et al. The probe serves to isolate the transducer from the high melt temperatures, which will usually run in the range of about 675.degree. to 825.degree. C., and to introduce a time delay between a transmitted pulse and echoes from inclusions located near where the pulse first enters the melt.
The probe will usually be in the form of a bar or rod, one end of which will be immersed in the melt and is known as the "working tip". And the other probe end is coupled to the transducer. It has been said that an ideal probe material should have the following properties:
(a) The material should have a constant low acoustic energy attenuation over the range of working temperatures at the frequencies used.
(b) It should be sound and homogeneous and have good resistance to thermal and mechanical shock.
(c) It should have a good resistance to attack by the molten metal. Any material which has the effect of reacting with the molten metal to form a protective film has the disadvantage that wetting of the immersed transmitting end of the probe by the molten metal will be materially reduced.
(d) It should have a low thermal conductivity.
(e) The acoustic impedance i.e. the product of density and the velocity of sound, should be of the same order as of the molten metal.
Apparently no material has been found which would fulfill all of these requirements.
Sintered rods made from titanium diboride and titanium carbide mixtures in 70/30 and 60/40 volumetric proportions have, for example, been examined by the prior art. With these rods, difficulty was encountered initially in obtaining rods of adequate soundness and in wetting the immersed ends of the rods to allow transmission of ultrasonic energy between the liquid aluminum and the probes. In attempts to effect wetting, the probes were immersed in the liquid aluminum under an inert atmosphere or argon. These attempts were not successful, even when the probe ends were capped with brazing metal before immersion. Greater success was obtained when the rods were capped with pure aluminum at high temperatures (e.g. 1200.degree. C.) under vacuum; these gave low attenuation and a very small loss of signal at the probe-aluminum interfaces. However, these benefits were lost when the probes were removed from the liquid metal and exposed to atmosphere. The probe end surfaces apparently oxidized so that on reimmersion full wetting did not occur and only a small proportion of the available signal was then transmitted into the metal.
A titanium alloy, Ti 317, containing 5% Al and 2.5% Sn (by weight) and obtainable with a single phase structure, was also examined by the prior art and found to resist erosion to a considerable extent. Material having a duplex (.alpha.+B) structure had a very high attenuation, so that it was only possible to transmit signals up to 2.5 MHz through a 2 ft..times.1 in. diameter rod. When converted to a single phase structure, it had a reasonable attenuation, though still higher than desirable. Also, experiments show that titanium does not become wetted until it has been immersed in molten aluminum for approximately thirty minutes.
After looking at titanium diboride-titanium carbide sinters and metallic titanium alloys as probe materials, at least one group came to prefer steel (0.26 wt.% carbon content) coated with a sprayed water-suspended Foseco Dycote 34 and tipped with a cap of silver solder. The silver solder accelerated the wetting so that the probes transmitted and received the available energy after approximately three minutes immersion. Once wetted, the probes could be removed from the liquid metal, allowed to cool and then replaced without undue loss of coupling efficiency. And the sprayed refractory coating prevented wetting of the sides of the probes and the introduction of stray vibrations into the liquid metal. It was a problem, however, that the steel tended to be dissolved in the aluminum melt. The probelm was tolerated by observing the amplitude of the reflected echoes, and when the amplitude fell to a predetermined level, the probes were removed, shortened and resoldered.
Hence, of various probe constructions that the prior art had looked at, each was effected by one or more of the following problems: wetting did not occur at all or only until the passage of some substantial amount of time after the probe was initially immersed in the melt; wetting did not occur after the probe was removed from the melt, exposed to the atmosphere and cooled, and then re-immersed; at operating temperatures, the probe material attenuated the ultrasonic signals to an undesirable degree; or the probe material was not chemically stable in molten aluminum.
It was against the foregoing background that this invention was made.